Cell membrane repair is an important aspect of normal physiology, and disruption of this process can result in pathophysiology in a number of human diseases including muscular dystrophy. We previously identified MG53 as an essential component of the cell membrane repair machinery. During the last funding period, we showed that critical cysteine residues and leucine-zipper motifs in MG53 participate in the nucleation process of cell membrane repair; non-muscle myosin type IIa (NM-IIa) acts as a molecular motor to transport MG53- containing vesicles toward the injury site; and PTRF enables anchoring and targeting of the MG53-repair machinery at the injured membrane. In addition to the intracellular action of MG53, injury to the cell membrane exposes a signal that can be detected by circulating MG53, allowing recombinant human (rh)MG53 protein to repair membrane damage when provided in the extracellular space. To develop MG53 as a potential therapeutic agent for treatment of muscle diseases, we have established the chemistry, manufacture and control (CMC) process for scale-up production of rhMG53 to support our pre-clinical and future clinical studies. Clearly, our translational approach cannot proceed without understanding the cellular processes that underlie the control of MG53 in circulation and the mechanistic action of extracellular MG53 in tissue protection. Specifically, a) how does circulating MG53 recognize the tissue injury site to facilitate protection against muscle injury an improve its survival under stress conditions? b) what are the cellular processes that control secretion, transport and clearance of MG53 in blood circulation? c) can sustained elevation of circulating MG53 provide beneficial effects to ameliorate the chronic process of muscular dystrophy? We have recently established a transgenic mouse model with controlled secretion of MG53 in circulation that can be used to examine the function for MG53 in protection against muscle injury in physiological and pathological conditions. This renewal application will focus on testing the hypothesis that MG53 in circulation protects muscle injury through membrane-delimited signaling pathways, and controlling MG53 secretion, transport and clearance from the blood stream may provide alternative ways for treatment of degenerative muscular diseases. We propose to test the concept that muscle is a paracrine organ that secretes MG53 for tissue protection and renal-mediated excretion of MG53 is a key factor that regulates MG53 levels in circulation. Our transgenic mouse model allows for tailored control of MG53 secretion, providing ways to examine the efficacy for circulating MG53 in ameliorating the pathology of muscular dystrophy in the mdx mice, as well as for evaluating the safety profile for sustained MG53 in affecting other organ functions.